The NTSC color television signal presently in use includes a wideband brightness or luminance (Y) signal and a modulated subcarrier signal of approximately 3.58 megahertz. The subcarrier signal is phase and amplitude modulated by 1 and Q chrominance signals which are modulated in quadrature so that the phases of the subcarrier represent the hue of an image portion and the ratio of the subcarrier amplitude to Y at that particular phase represents the saturation of that hue. A monochrome receiver yisibly reproduces only the Y component.
The usual color television receivers include two or three color demodulators for synchronously recovering the color difference signals which then can be added to the Y signal in a matrix circuit for developing the red, blue and green representative signals to be reproduced by the cathode ray tube. Other receivers include direct demodulators for directly developing the red, blue and green color representative signals, thereby avoiding the separate recovery and combination of the brightness signal with the demodulated color difference signals.
In either of these types of receivers, however, it is necessary to provide a properly phased reference signal at the subcarrier frequency in order to produce color representative output signals at the proper hues. In the NTSC system, this is accomplished by including in the television signal bursts of a reference signal of the same frequency as the color subscarrier and having a particular phase relationship with the different phases of the subcarrier representing the different hues. These reference signal bursts are recovered at the receiver by a gating action and are applied to an automatic frequency control circuit associated with the local oscillator at the receiver. The recovered burst signal and the output signal from the oscillator are compared to develop a control signal which is utilized to control the oscillator so that its output signal is a continuous wave of the proper frequency and phased to be used as a demodulating reference signal for the color subcarrier.
Since the burst signal theoretically occupies a position having a precise phase relationship with the modulated subscarrier signals, a local oscillator which is phased-locked to the burst signal, should provide an accurate reference signal for demodulating the correct hues of the transmitted signal. Thus, once a hue setting is made by the operator of the television receiver to suit his particular taste, theoretically it is not necessary to change that hue setting thereafter.
In actual practice, however, incorrect hue in the reproduced color picture has been a problem from the beginning of color broadcasting. The user can adjust the hue control at the reaceiver for optimum color reproduction, but this adjustment is only temporary. It has been found that the transmitted burst does not always occupy the same phase relationship with the modulated color difference signals from station to station, from program to program on a given station, or even from camera to camera on a given program. Each of these variations requires a readjustment of the hue control. For most objects, these shifts in hue or color are not detectable by the viewer since he has no reference based on previous information to make an exact comparison.
Certain objects, however, such as green trees or blue sky provide a convenient reference. However, the most critical and common color reference in the reproduced picture that the viewer can rely upon is the color of flesh tones. The viewer detects errors in the color reproduction of flesh tones since he has a pre-established reference for flesh tones in his own mind. There is some flesh color in the vast majority of scenes, so that hue errors in flesh colors are particularly annoying. Therefore, an automatic hue control should provide an automatic flesh color control and, if possible, provide a minimum amount of distortion for other colors, particularly for green and blue (since these colors also have preestablished references in the minds of the viewers).
Since the hue of flesh colors lies very near the positive 1 axis (+1), it is desirable to dynamically adjust the hue of all colors that are near the phase of the positive 1 axis to the phase of that axis or the axis of flesh; so that flesh color will always appear correct, even if there is a phase shift of the chrominance signal relative to the regenerated reference subcarrier. If in the process of doing this, the hue (phase) of other colors that are near the positive 1 axis is changed slightly, it is unlikely that the viewer will notice this; because there is no common video content reference for the viewer to remember and these colors are not such as have a predetermined memory relationship for a viewer. The exact angle of flesh tones is just slightly off the positive 1 axis, five to ten degrees toward the red axis of a standard color phase diagram, regardless of the race of the person viewed. Therefore, saturation not hue is the main variable in flesh color. The range of flesh saturation is from ten to seventy percent as determined by extensive observation of color television programming.
The two most common approaches to automatic hue control are (1) changing the decoding angles to emphasize the 1 axis colors, and (2) reducing the Q component of the chrominance signal when the chrominance signal is near the positive 1 axis.
The advantage of a method which changes the decoding angles is economy, and this approach also can be implemented by static adjustments. However, changing the phase angles to emphasize 1 axis colors tends to make the color reproduction more like a two-color system and produces relatively large color distortions in both hue and saturation. The most noticeable distortions are usually in green, which normally is very near the angle of the negative Q axis. In this type of system, the reproduced green color gets a strong blue content or becomes de-saturated or both. Since the viewer remembers the true color of vegetation, the bluish cast given to scenes with vegetation, such as a forest scene, a football or baseball game, is quite noticeable. A more consistent rendition of flesh color is achieved at the expense of faithful green reproduction.
The second approach which has been used is one which automatically reduces the Q component of the chrominance signal whenever the chrominance signal is within a predetermined angle about the the positive 1 axis. This approach usually results in an economic penalty, because of the need for bandpass filters centered at the color subcarrier frequency at approximately 3.58 megahertz and also usually involves relatively comples circuitry. In addition, there is a saturation error associated with such an approach resulting from the collapsing of the Q component of the signal to the 1 axis. Such saturation errors can be minimized by introducing additional circuit complexity.
It is desirable to minimize color errors in an NTSC color television receiver in a way which is not subject to the disadvantages of the systems discussed above and to do so economically.